1. Technical Field
The invention relates to a turbine, in particular a Francis turbine comprising a housing, a guide apparatus and a closed rotor wheel having an outer cover disk and an inner cover disk for delimiting a main flow, each of which are formed in a respective sealing region as a contactless seal with a small radial play towards the housing in order to only permit small leakage currents in front of the rotor wheel inlet which do not flow through the rotor wheel for energy conversion.
2. Description of the Prior Art
Water turbines such as Francis turbines are, as a rule, coupled to a generator for power generation and are therefore operated with a rotational speed corresponding to the main frequency. Very high efficiencies are achieved at the design point during full load operation, whereas during partial load, a poorer flow through the rotor blades takes place. When the guide apparatus is adjusted for partial load, a co-rotating swirl arises at the rotor wheel outlet which produces a "pig-tail" in the center which can in some conditions produce noise and beats. This is often remedied by controlled, forced ventilation so that, on the one hand, a desired damping is provided and, on the other hand, under-pressure is retained. The air introduced by this fills up a part of the cross-section behind the rotor wheel.
Today, there are systems with which the ventilation takes place essentially along the rotor wheel shaft by blowing air in through the inner cover disk into the center of an emitting flow ring--see P. Doerfler, Design Criteria for Air Admission System in Francis Turbines, IAHR Symposium, September 1986, Montreal, Canada. Other systems have a fixed middle tube in the emitting flow ring with a hollow tripod protruding into the flow and provided with openings in order to add compressed air to the flow--see H. Grein, Schwingungserscheinungen in Francisturbinen--Ursachen und Gegenma.beta.nahmen (Oscillation Effects in Francis Turbines--Causes and Countermeasures), Escher Wyss Mitteilungen 1/1981-1/1982. In all these cases compressors are necessary to produce larger compressed air flows. Moreover, installations such as tripods protruding into the flow represent flow resistances which also result in loss at the operational point with the best efficiency. Furthermore, in smaller turbines the cost of periphery equipment, such as compressors, is generally too high to be able to justify them.